WHEELS 'N CUSTOMS TECHNICAL GUIDE: OPF / GPF FILTERS, EXHAUST SOUND & APK LEGISLATION

Anyone moving today from a sporty petrol car from before 2018 to a more modern performance model – such as a Volkswagen Golf 8 R, BMW M3 (G80) / M2 (G87), Audi RS3 (8Y) or Mercedes-AMG A45 S – will immediately notice a striking difference: the exhaust sound is significantly quieter, the characteristic shift bangs (DSG pops) have virtually disappeared and the raw exhaust note sounds muted. The cause of this is not a software choice by the manufacturer, but a physical component in the exhaust line: the OPF filter (Otto Partikel Filter), also known as the GPF (Gasoline Particulate Filter).

In the tuning world this component raises huge questions: "What exactly does an OPF filter do in a petrol car?", "Why do cars built after September 2018 sound so much tamer?", "Is an OPF delete legal for the APK inspection and particle counter test?" and "How do you achieve an aggressive sound with an OPF-back sports exhaust system without error messages?" In this technical guide, the exhaust specialists at Wheels 'N Customs explain in detail the operation, sensor pressures, APK rules and legal tuning solutions.

From September 2018, the stringent Euro 6d-TEMP emissions standard came into effect within the European Union. Modern direct-injection petrol engines (GDI / TSI / TFSI) inject fuel at extremely high pressure directly into the combustion chamber. While this results in lower fuel consumption and more power, it also creates microscopic soot particles (particulate matter) comparable to the emissions of traditional diesel engines.

Many enthusiastic DIYers think that physically cutting out or gutting the OPF filter is the quickest way to get the sound back. However, with modern vehicles this immediately leads to serious electronic issues:

So how do you get a fantastic, sporty soundtrack from your car without hassle with warning lights or warranty issues? The answer is a certified OPF-Back performance exhaust system from specialists such as Milltek Sport, Remus or Akrapovic:

• The section after the filter: An OPF-back system leaves the original downpipe, the primary catalytic converter and the complete OPF filter with all sensors 100% original and untouched. The exhaust is mounted directly behind the flange of the OPF filter.
• Larger Pipe Diameters & Freer Flow: By removing the restrictive factory centre silencer and using larger pipe diameters (for example 76 mm / 3 inch T304L stainless steel) in combination with absorption rear silencers, the remaining sound is maximally amplified. The car sounds deep, dynamic and lively again.
• Electronic Exhaust Valves (OEM Valve Control): Quality systems from Milltek Sport integrate seamlessly with the vehicle’s original electric actuators. In the 'Comfort' mode the valves close for relaxed cruising; in the 'Sport' or 'Race' mode the valves open fully for an unfiltered sound.
• No Remapping Required: Because nothing changes in the emissions path, the ECU registers zero deviations. You do not need any software modification (reflash), the engine management light stays off and the factory warranty on the engine management remains intact.

The days when a car with a removed particulate filter could easily ‘fool’ the periodic inspection are definitely over. Since the introduction of the new emission guidelines, inspection stations in the Netherlands and Belgium check with advanced equipment:

• The Particle Counter Test (particle measurement): Where previously only opacity or CO value was checked, the official particle counter measures the exact number of soot particles per cubic centimetre of exhaust gas. Has the OPF filter been removed or gutted? Then the particle count rises from a few tens of thousands to well over 1,000,000 particles per cm³. This results in an immediate and irrevocable APK failure.
• Visual Inspection Requirement: According to RDW regulations, any emission control system recorded on the vehicle registration certificate or in the type-approval register must be physically present, undamaged and mounted without leaks. A cut-through or gutted filter canister leads directly to rejection.
• WOK Status during police checks: In the Netherlands and Germany, the police carry out intensive technical checks with mobile measuring equipment along known motorways and at meets. Are you driving with a manipulated OPF filter? Then the registration certificate will be confiscated on the spot (WOK status: Wachten Op Keuren – Waiting For Inspection), you will receive a heavy environmental fine, and the vehicle may only be presented to the RDW again after a costly restoration to original condition.
• Heat Damage & Warranty Exclusion (Article 10 T&Cs): In accordance with Article 10 of our Terms and Conditions, warranty on bumpers, diffusers and styling parts is void at all times if damage occurs due to abnormal heat spikes as a result of a removed OPF filter, crackle maps or illegal 'pops & bangs' software.